1. Field of the Invention
The embodiments discussed herein relate to a current detection device for a power semiconductor element, and particularly to a current detection device for a power semiconductor element including a main element in which a main current flows and a current sense element for detecting the current of this main element.
2. Background of the Related Art
An electric power converter device, such as an inverter device for driving an electric motor and an uninterruptible power supply device for a computer, switches a power semiconductor element to perform electric power conversion. The electric power converter device needs to protect the device against a short circuit trouble that occurs suddenly outside the device, in order to prevent destruction of the device. To include this function in the electric power converter device, the electric power converter device has a function for continuously detecting current that flows in a semiconductor element of the electric power converter device.
FIG. 2 is a circuit diagram illustrating a power semiconductor element and a conventional first current detection circuit. This circuit diagram illustrates an example in which a power semiconductor element 100 of an electric power converter device is an insulated gate bipolar transistor (IGBT). Note that the power semiconductor element 100 is not limited to the illustrated IGBT, but may be other insulated gate semiconductor elements, such as an insulated gate field-effect transistor.
The power semiconductor element 100 includes an IGBT 101 which is a main element in which a main current flows and a sense IGBT 102 which is a current sense element for detecting the current of this IGBT 101. This sense IGBT 102 has the same structure as the IGBT 101 and has a smaller size than the IGBT 101. The power semiconductor element 100 has a configuration in which the IGBT 101 and the sense IGBT 102 are connected to each other to have a common gate terminal 103 and a common collector terminal 104. The gate terminal 103 is connected to a drive circuit of this power semiconductor element 100, and the collector terminal 104 is connected to a power supply via a load, such as an electric motor. Also, in the power semiconductor element 100, an emitter terminal 105 of the IGBT 101 and an emitter terminal 106 of the sense IGBT 102 are configured independently. The emitter terminal 105 of the IGBT 101 is connected to a ground 107, and the emitter terminal 106 of the sense IGBT 102 is connected to one end of a resistor 108, and another end of the resistor 108 is connected to the ground 107.
In a control state in which the power semiconductor element 100 is turned on, the current that flows in the emitter terminal 105 of the IGBT 101 is greater than and proportional to the current that flows in the emitter terminal 106 of the sense IGBT 102. In this case, the current that flows in the emitter terminal 106 of the sense IGBT 102 flows in the resistor 108 as it is, and thus this resistor 108 generates a voltage drop that is proportional to the current that flows in the emitter terminal 105 of the IGBT 101. A circuit (not depicted) for measuring this voltage drop is connected to the resistor 108, and the circuit measures the voltage drop to indirectly detect the current that flows in the IGBT 101.
In current detection by the configuration of the above FIG. 2, there are two known problems in deciding the size of the resistor 108, i.e., a first problem relevant to current detection accuracy and a second problem relevant to erroneous operation due to disturbance noise.
The first problem relevant to current detection accuracy is that, as the resistance value of the resistor 108 increases, its voltage drop increases, and thus the voltage between the gate and the emitter of the IGBT 101 becomes significantly different from the voltage between the gate and the emitter of the sense IGBT 102. If the voltages between the gates and the emitters of the IGBT 101 and the sense IGBT 102 are different from each other, accuracy of the current rate between the current that flows in the IGBT 101 and the current that flows in the sense IGBT 102 deteriorates, and thus current detection accuracy deteriorates.
The second problem relevant to erroneous operation due to disturbance noise is that, as the resistance value of the resistor 108 decreases, its voltage drop decreases, and the above current detection accuracy improves, but a threshold value to determine overcurrent at the time of short circuit becomes small, and thus erroneous operation due to disturbance noise is likely to occur.
The above first and second problems and means for solving these problems are already proposed (for example, Japanese Laid-open Patent Publication No. 2012-186899 (paragraphs [0017] to [0041] and FIG. 1)), and thus the current detection proposed here will be described.
FIG. 3 is a circuit diagram illustrating a power semiconductor element and a conventional second current detection circuit. Note that, in FIG. 3, components that are same as or equivalent to what are described in FIG. 2 are denoted with the same reference signs.
In the power semiconductor element 100, a free-wheeling diode 109 is connected in inverse parallel to a collector terminal 104 and an emitter terminal 105 of an IGBT 101. The emitter terminal 105 of the IGBT 101 is connected to a ground 107 having a ground potential.
The power semiconductor element 100 is configured to be controlled by a drive control circuit 110. The drive control circuit 110 includes a drive circuit 120 connected to a gate terminal 103 of the power semiconductor element 100, an overcurrent determination circuit 130, and a current detection circuit 140 that detects current that flows in a sense IGBT 102.
The drive circuit 120 includes a driver 121 and direct current power supplies 122 and 123, and the driver 121 drives and controls the power semiconductor element 100 by using the direct current power supply 122 as a drive power supply. A negative electrode of the direct current power supply 122 is connected to a common connection part 111 of the drive control circuit 110 to provide a reference potential of the drive control circuit 110. With regard to the direct current power supply 123, a positive electrode is connected to the ground 107 of the ground potential, and a negative electrode is connected to the common connection part 111 of the reference potential of the drive control circuit 110.
The current detection circuit 140 includes a PNP transistor 141 and a resistor 108. With regard to the PNP transistor 141, an emitter terminal is connected to an emitter terminal 106 of the sense IGBT 102, and a collector terminal is connected to one end of the resistor 108, and a base terminal is connected to the ground 107 of the ground potential. Another end of the resistor 108 is connected to the common connection part 111 of the reference potential of the drive control circuit 110.
The overcurrent determination circuit 130 includes a comparator 131 and a direct current power supply 132. With regard to the comparator 131, one of input terminals is connected to a connection point between the collector terminal of the PNP transistor 141 and the resistor 108 in the current detection circuit 140, and the other of input terminals is connected to a positive electrode of the direct current power supply 132. A negative electrode of the direct current power supply 132 is connected to the common connection part 111 of the drive control circuit 110. Thereby, the comparator 131 compares an electric potential difference Vs of the resistor 108 relative to the common connection part 111 with the voltage of the direct current power supply 132, and determines that overcurrent has flowed in the IGBT 101 of the power semiconductor element 100 when the electric potential difference Vs exceeds the voltage of the direct current power supply 132.
According to the current detection circuit 140 of this drive control circuit 110, when the power semiconductor element 100 is turned on to flow current in the sense IGBT 102, the PNP transistor 141 of the current detection circuit 140 is also turned on. When the PNP transistor 141 is turned on, the voltage between the base and the emitter of the PNP transistor 141 becomes approximately equal to forward voltage drop of one piece of diode. Hence, the emitter terminal of the sense IGBT 102 is substantially fixed to the electric potential of the forward voltage drop of one piece of diode relative to the ground potential of the ground 107, and the voltages between the gates and the emitters of the IGBT 101 and the sense IGBT 102 are not substantially different from each other. Accuracy of the current rate between the current that flows in the IGBT 101 and the current that flows in the sense IGBT 102 does not change significantly, and thus the current detection accuracy does not deteriorate, so as to solve the above first problem.
Also, the above second problem is solved in the same way, because the reference potential of the resistor 108 is set to a lower electric potential than the ground potential of the ground 107 by using the direct current power supply 123, allowing a larger voltage drop of the resistor 108.
Here, the power supply for operating the current detection circuit 140 is −V2 to (V1−V2), where V1 is a voltage of the direct current power supply 122 illustrated in FIG. 3, and V2 is a voltage of the direct current power supply 123. Hence, the current detection circuit 140 needs to be operated from a negative voltage, which imposes a problem of complicated system design on the drive control circuit 110.